1. Field of the Invention
This invention relates to gauges and, more particularly, to gauges for measuring the pitch diameter of internally or externally threaded cylindrical or truncated cone-shaped members.
2. Description of the Prior Art
Threaded end connectors are commonly used to join together a variety of objects. If the threads are not formed within acceptable tolerances, the connection will not be as tight and sturdy as expected. This is a particular concern in certain industries, such as the petroleum industry which connects many lengths of pipe together underground and demands that the threaded connections not fail. In view of the criticality of these connections, inspectors routinely check a variety of characteristics about the threaded ends before the pipes are joined together and inserted into a drill hole.
One of the critical characteristics is the pitch diameter of an internally or externally threaded pipe, whether it is a straight or tapered threaded connection. The pitch diameter is particularly important since the pitch line is the location at which the adjacent threads actually touch each other and form the connection. The pitch line runs through the slope between the crest and the root of the threads. Various standards have been established for determining whether the pitch diameter of a particular diameter pipe and style of thread is acceptable. For example, the American Petroleum Institute has instituted a series of thread standards, such as standard 5B for pitch diameter. These standards tell the inspector the expected pitch diameter of a particular threaded connector at a particular distance from the end of the pipe. For example, L1 is the length back from the end of the pipe for measuring pitch diameter of an eight round thread and E1 is a similar standard for buttress threads. Measuring the threads at the end of the pipe is particularly important since that is the area most likely to be damaged.
A variety of devices have been used in the past for measuring pitch diameter. For example, the taper gauges sold by Allen Gauge & Tool Company, the assignee of the present application, under Model No. 5600 series and Model No. 5572 have been used to measure pitch diameter. The problem with using a taper gauge is that various errors in the taper or pitch of the threaded connector or even in the formation of the threads will, while being an acceptably formed thread, introduce errors in the actual pitch diameter measured. Helix error is also a particular problem with taper gauges using a two point contact. In addition, the ring and plug method has been used, but this arrangement is expensive and unwieldy to use, requiring separate, precisely machined ring/plug pairs for numerous thread configurations and diameters.
The three wire method of measuring pitch diameter is well known. In accordance with this method, a measurement is made of the distance of a chord through the center of a thread circle over three wires of exact and equal diameters which are placed into the thread groove. Subtraction of the known wire diameter from the measured reading gives the exact pitch diameter of the threaded member since the wires contact the slope of the threads at the pitch line. See, for example, U.S. Pat. Nos. 2,548,917, 2,937,452, 4,335,518 and 4,480,388. These arrangements provide a satisfactory measurement of the pitch diameter and will compensate for a variety of errors in the thread formation. However, these devices are unwieldy to use and do not readily lend themselves to portability. Other devices have been used to measure pitch diameter, including, the devices shown in U.S. Pat. Nos. 1,423,339, 1,661,857, 2,937,453, 3,271,872, 3,277,578, 3,537,184, 3,827,154, 4,202,109, 4,524,524, 4,553,337, 4,567,670, 4,590,678 and 4,672,750. However, each of these devices fails to correct one or more of the forms of error which could cause a deviation in the actual pitch diameter measured. Such errors include error of the taper of the thread, error in the pitch, error in thread height, error in thread form, and helix error in not properly spanning a perpendicular line across the thread. These errors are particularly pronounced in devices which contact the top of the threads, such as the devices shown in U.S. Pat. Nos. 4,524,524 and 4,567,670.
Accordingly, it is an object of the present invention to provide a gauge for measuring pitch diameter which will compensate for all of the possible errors identified above. It is also an object of the present invention to provide such a gauge which is portable and can be readily adjusted for measuring threaded connectors of various diameters, including large diameters. In addition, it is an object to provide such a gauge which can be used to measure the pitch diameter on an externally or internally threaded connector and on straight or tapered threads. It is also an object of the present invention to use the principles behind the three-wire method to contact the slopes of the threads and provide a precise and accurate measurement of the pitch diameter at the pitch line.